This proposal seeks effective combination therapies that maximize GIST response to KIT/PDGFRA inhibition by concurrently targeting the biologically key MEK/MAPK pathway. Most GISTs express mutant, constitutively activated forms of the KIT or PDGFRA, and we have shown that these formerly untreatable cancers can be palliated in 80% of patients by oral single-agent therapy with imatinib mesylate. However, patients responding to imatinib have persistent measurable disease and generally develop resistance within two years of starting treatment. Therefore, more effective and broader-spectrum therapies are urgently needed. Notably, our preliminary studies show that KIT/PDGFRA imatinib resistance mechanisms vary from patient to patient, and also between metastatic lesions in a given patient, but uniformly rely upon MEK/MAPK signaling to support cell proliferation. In Aim 1, by studying MEK/MAPK signaling and response mechanisms, we will develop clinically-relevant biomarkers and - most importantly - we will identify alternate MEK-dependent therapeutic targets which might have greater specificity, in GIST, compared to MEK. In Aim 2, we will characterize mechanisms of MEKi resistance, since such studies are likely to identify biologically essential regulatory nodes in MEK/MAPK-pathways, which - like those found in Aim 1 - will be candidates as biomakers and therapeutic targets in GIST clinical trials. The collective studies in Aims 1-2, by revealing the scope of MEK/MAPK signaling in GIST, will provide the understanding needed to design more effective and less toxic clinical trials. In Aim 3 we evaluate combination therapies with imatinib and MEKi, as a strategy to inhibit downstream signals from the varied gain-of-function KIT mutations each imatinib-resistant patient, while maintaining imatinib inhibition of nonprogressing GIST subclones. This will be accomplished through a phase l/II clinical trial of the MEK inhibitor, MEKI62, combined with imatinib, in patients showing progression of metastatic GIST on imatinib or sunitinib. Through these studies, we will translate the basic science proposed in this SPORE through to clinical application.